Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using autonomous underwater vehicles (AUVs) that carry appropriate seismic sensors.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which are especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14. The streamers may have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward and penetrates the seafloor 20 until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver(s) 11 on the streamer(s) 12. Based on the data collected by the receiver(s) 11, an image of the subsurface is generated by further analyses of the collected data. The seismic source array 16 includes plural individual source elements, which may be distributed in various patterns, e.g., circular, linear, at various depths in the water.
However, this traditional configuration is expensive because the cost of streamers is high. New technologies deploy plural seismic sensors on the bottom of the ocean (ocean bottom stations) to improve the coupling. Even so, positioning seismic sensors remains a challenge.
Other technologies use permanent receivers set on the seafloor, as disclosed in U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference. In this case, the seismic sensors are attached to a heavy pedestal. A station holding the sensors is launched from a vessel and arrives, due to its gravity, at a desired position. The station remains on the seafloor permanently. Data recorded by sensors is transferred through a cable to a mobile station. When necessary, the mobile station may be brought to the surface to retrieve the data.
Although this method provides a better coupling between the seafloor and the sensors, the method is still expensive and not flexible because the station and corresponding sensors are left on the bottom of the ocean. Further, positioning the sensors is not straightforward.
An improvement to this method is described, for example, in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, a sensor is removably attached to a pedestal together with a memory device. After recording the seismic waves, the sensor and memory device are instructed by a vessel to detach from the pedestal and rise to the ocean surface for pick-up by the vessel.
However, this configuration is not very reliable because the mechanism maintaining the sensor connection to the pedestal may fail to release the sensor. Also, the sensor and pedestal may not reach their intended positions on the seabed. Further, leaving the pedestals behind increases ocean pollution and the survey price, which are both undesirable.
Accordingly, it would be desirable to provide systems and methods that provide an inexpensive and non-polluting device for reaching the bottom of the ocean, recording seismic waves and resurfacing for data collection.